The present invention relates to a picture printing apparatus, especially to a laser beam printer for writing-in information using laser beam scanning.
A standard laser beam printing apparatus, which prints information, including characters and pictures, writes information represented by a pixel matrix using a laser beam scanning method. And, usually, a laser beam scans a photo-conductor with a predetermined beam spot size for a predetermined pixel density to be printed.
On the other hand, printed pixel density changing or neutral grade picture printing is required corresponding to the diversification of information to be represented or the need for high-definition picture printing.
Therefore, it is desirable to be able to change a beam spot size in a laser beam scanning system to a required size. As a method of changing a spot size, there is a well known method of changing a printed spot size by changing the power of the laser beam. However, since the beam spot size itself is not changed, but only the power exposed on a photoconductor by the laser beam in accordance with this method, the gradation representation in a fine part of a picture makes it difficult to display a high-resolution picture.
Further, in JP-A-196023/1991, there is disclosed a method of changing a pixel size printed on a scanning plane by using a combination of adjusting the exposure time and using a light element, provided in a beam path, for changing the beam spot size in a specified direction. And, a method of obtaining a desired beam spot size by using a liquid crystal shutter in a beam path is disclosed in JP-A24771/1987.
However, those methods have problems, such as the necessity of providing a specific light element for modulating the beam spot size and the large size of the resulting light system, laser power losses, low speed in the spot size modulation, etc. In order to solve the above-mentioned problems, a semiconductor laser element in which the beam spot size and the laser power are separately controlled has been proposed in JPA-97580/1994.
The newly devised semiconductor laser element, while solving some problems, also has the following problem, that is, since a light wave guide part is commonly used in the element, the two kinds of controls for the beam spot size and the laser power interrelate with each other in a non-linear manner, which prevents a high-resolution picture display from being obtained.